1. Field of the Invention
This invention broadly relates to a method useful for recovering a Group VIII transition metal from a liquid, and in particular from liquids containing the transition metal in solution at a low concentration. This invention particularly relates to a method for recovering rhodium from its liquid solutions, such as from the aldehyde product of an olefin hydroformylation process.
2. Description of Related Art
Processes using homogeneous transition metal catalysts such as rhodium are well-known. For example, transition metal catalysts are used in processes for hydrogenating unsaturated compounds, such as copolymers of a conjugated diene and co-polymerizable monomers as described in U.S. Pat. Nos. 4,464,515 and 4,503,196, for carbonylating methanol to acetic acid, for oligomerizing olefins, for hydrocyanating butadiene to adiponitrile, for decarbonylating aldehydes and for hydrosilylating olefins. Particularly illustrative of such homogeneous catalysis systems is the catalytic hydroformylation of olefinic compounds with carbon monoxide and hydrogen to produce aldehydes. In such system, the rhodium catalyst generally is stabilized with a complex-forming agent, referred to in the art as a ligand, although use of a simple rhodium catalyst without such additional stabilizing agents also is known.
In one known arrangement, the rhodium complex catalyzed hydroformylation process is carried out in a non-aqueous hydroformylation reaction medium, the reaction-medium contains an organic solvent and both organic solvent-solubilized catalyst complex and solubilized free ligand, i.e., ligand not tied to bound to the rhodium catalyst complex. Organic solvents which do not interfere with the hydroformylation process are employed. Suitable organic solvents include those used in known Group VIII transition metal catalyzed processes such as alkanes, ethers, aldehydes ketones, esters, amides, aromatic hydrocarbons and mixtures of different organic solvents. Processes representative of such non-aqueous hydroformylation process are described in U.S. Pat. No. 3,527,809; U.S. Pat. Nos. 4,148,830 and 4,247,486; U.S. Pat. No. 4,260,828; U.S. Pat. No. 4,283,562; U.S. Pat. No. 4,306,087; U.S. Pat. No. 4,400,548; U.S. Pat. No. 4,429,161; U.S. Pat. No. 4,482,749; U.S. Pat. No. 4,491,675; U.S. Pat. No. 4,528,403; U.S. Pat. No. 4,593,011; U.S. Pat. No. 4,593,127; U.S. Pat. No. 4,599,206; U.S. Pat. No. 4,633,021; U.S. Pat. No. 4,668,651; U.S. Pat. No. 4,694,109; U.S. Pat. No. 4,716,250; U.S. Pat. No. 4,717,775; U.S. Pat. No. 4,731,486; U.S. Pat. No. 4,737,588; U.S. Pat. No. 4,748,261; European Patent Applications Publication Nos. 96,986; 96,987; and 96,988 (all published Dec. 28, 1983); PCT Applications Publication Nos. WO 80/01690 (published Aug. 21, 1980) and WO 87/07600 (published Dec. 17, 1987) to name a few. In these systems, product may be recovered by selective vaporization of the aldehyde under reduced pressure and at temperatures below about 150.degree. C., preferably below about 130.degree. C.
It also is known to use water or a similar polar solvent such as methanol as the catalytic reaction medium when hydroformylating olefins. One such process is described in U.S. Pat. No. 4,248,802, wherein water-soluble complexes of rhodium and certain salts of sulfonated triarylphosphines are used as the hydroformylation catalyst. Separation of aldehyde product from catalyst in this process is facilitated by the mutual immiscibility of product and catalyst solution. However, this approach typically requires more severe reaction conditions than the non-aqueous systems.
A new process also has been developed for recovering by a simple phase separation the aldehyde products which are produced when hydroformylating olefinic compounds in a non-aqueous reaction medium containing an ionic phosphorus ligand separate from the rhodium catalyst components. This process is described in copending application Ser. No. 218,911 entitled Process for Catalyst-Aldehyde Product Separation filed on Jul. 14, 1988 in the names of A. G. Abatjoglou, R. R. Peterson and D. R. Bryant.
In the case of both non-aqueous (non-polar) and aqueous (polar) hydroformylation processes, various rhodium-containing liquid streams may be generated that are not recirculated to the hydroformylation reactor. For example, in hydroformylation processes relying on phase separation to recover product aldehyde it is not uncommon for some of the rhodium to be removed with the aldehyde. The high cost and limited supply of rhodium make it imperative that rhodium loss via the aldehyde product be reduced to the lowest possible level. In fact, for economic operation, the rhodium concentration in the product aldehyde preferably should be reduced to less than about 50 parts per billion (ppb) and more preferably to less than about 20 ppb.
It is an object of the present invention, therefore, to provide a method for recovering Group VIII transition metals, such as rhodium from a liquid. It also is an object of the present invention to provide a method suitable for recovering Group VIII transition metals present at a very low concentration from a liquid. It is a particular object of the present invention to provide a method for recovering rhodium from a liquid, such as a hydroformylation aldehyde product.